1. Field of the Invention
The present invention relates to a semiconductor optical device integrated with a photo detector, particularly, a light-emitting device capable of monitoring its optical output by front light.
2. Related Prior Art
Japanese Patent Application published as 2002-299751 has disclosed a semiconductor laser diode integrated with an optical modulator type of electro-absorption device. In this application, the photodiode monitors backward light emitted from the back facet of the optical resonator of the laser diode. The laser diode of this application is currently driven by the driving signal after electrically correcting the output from the photodiode by a non-linear amplifier.
In general, an optical beam emitted from the back facet and that emitted from the front facet do not always keep their relative condition. That is, a ratio of both beams varies as the driving condition of the laser diode. Therefore, it is preferable to monitor the light emitted from the front facet.
Another Japanese Patent Application published as 2001-208939 has disclosed an optical module using a composite device, in which a laser diode and a photodiode for monitoring light emitted from the laser diode are integrated. The integrated device is mounted on a heat sink, where an optical fiber is also mounted thereon. The tip surface of the optical fiber is inclined to the optical axis thereof, accordingly light emitted from the laser diode enters the photodiode reflected by the tip surface of the optical fiber.
In the optical configuration above, although the photodiode receives the light emitted from the front face of the laser diode, an additional optical alignment between the tip surface of the optical fiber and the photodiode is necessary.
Still another Japanese Patent application published as 2002-223027 has disclosed an optical module, which includes a photodiode mounted on a substrate and a laser diode optical coupling with an optical waveguide. The photodiode receives light leaked from the core portion of the optical waveguide. In this module, the photodiode monitors light emitted from the laser diode without any optical splitting device.
In this prior art, the portion of the light emitted from the front facet of the laser diode without the optical alignment between the optical fiber and the photodiode is omitted, nor any optical splitter to split the front light. However, the light leaked from the core of the optical fiber is sensitive to the structure thereof and the driving condition of the laser diode.
Therefore, one of objects of the present invention is to provide a semiconductor optical device capable of monitoring light emitted from the front facet of the optical device in stable.